1. Field Of The Invention
The present invention relates generally to graphic recording apparatus of the type wherein an elongated record medium or sheet is moved along a first path relative to a sheet-marking means or element, and wherein the latter is moved over a second or sheet marking path, which is substantially normal to the path of movement of the sheet, in order to cause the marking element to produce on the sheet a series of recorded transverse traces which make up a recorded image representing data supplied to the apparatus. The invention relates particularly to such apparatus wherein the marking element is in the form of a deflectable beam or other low inertia means, such as a laser beam or the beam of a cathode ray tube.
2. Description Of The Prior Art
In many forms and applications of recording apparatus of the type noted above, the quality and fidelity of the recorded images are dependent upon the steadiness or constancy of the velocity at which the record sheet is moved along its path relative to the marking element. An example of such recording apparatus wherein this constancy of sheet velocity is important is the recording apparatus which is arranged to operate in the so-called gray scale mode to produce so-called gray scale records or recorded images. In such apparatus, the intensity with which the marking element marks the sheet is controlled or modulated as the element is moved back and forth along its marking path, and the element and sheet velocities are so chosen that the recorded record appears as a varying density image or series of images which represent the data applied to the apparatus. An example of an important application for this form of apparatus is that in which the apparatus is used to produce recorded images of objects which are scanned by the known ultrasound scanning or imaging devices. A specific example of this form of recording apparatus is that wherein the marking element is the beam of a fiber optics cathode ray tube, wherein the record sheet is a light-sensitive sheet which is moved along its path of travel across and in contact with the fiber optics strip of the tube, and wherein the beam is moved back and forth along the fiber optics strip and is intensity modulated in accordance withe the data produced by the scanning device.
Although the record sheet moving or driving arrangements which have been developed for use with the above-described recording apparatus are capable of moving the record sheet at a sufficiently constant average velocity, those arrangements have been found to be incapable of preventing short term sheet velocity changes or velocity errors which occur due to the inevitable mechanical and electrical characteristics of the driving arrangements. Those short term velocity errors are such as to make the sheet movement sufficiently irregular that the amount of recording light falling on a given area of the sheet per unit time fluctuates to the extent that corresponding differences in the density of the final, developed recorded images are produced. Those density differences typically manifest themselves as strips across the sheet, which are generally regarded as undesirable. Attempts to smooth out the irregularities in the sheet movement to minimize the resulting image density differences by improving the sheet driving means have generally proved to be unsuccessful, due to the inherently high inertia of the mechanical parts involved.